Patent GB-404650-A (GB)
Title: Improvements in and relating to protective systems for electric circuits
IPC: C10B1/04
Filed: 1932-07-13
Abstract: 404,650. Protective cut-out arrangements. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.- (Assignees of Fitzgerald, A. S. ; Warwick Road, Wynnewood, Pennsylvania, U.S.A.) July 13, 1932, No. 19837. Convention date, July 13, 1931. [Class 38 (v).] In a protective arrangement of the type described in Specification 274,070, [Class 38 (v), Electric switches &c.], in which highfrequency carrier currents are transmitted between transmitters and receivers at different points of the protected circuit, the transmission of such currents being controlled in accordance with the instantaneous directions of the current at different points, e.g., at the two ends of a section of the protected circuit, the amount of energization of the transmitter is, according to the present invention, made independent of the magnitude of the current in the protected circuit. The source of current from the transmitter is, in the arrangement described, obtained from a local D.C. source through converting apparatus of the series condenser inverter type described in Specification 347,474, [Group XXXV], the inverter being controlled in accordance with the phase of current at the corresponding point of the protected circuit. The inverter as employed includes an excitation lock-out feature of the type described in Specification 398,498, [Group XXXV.]. As shown in Fig. 1 applied to a 3-phase earthfault-responsive arrangement, the tripping coils 17 of circuit-breakers 15, 16 at the ends of a protected section 10, 11, 12 are controlled by polarised relays 18 each fitted with a polarising winding 20, an operating winding 21 and a restraining winding 22. The operating winding is fed from the parallel connected secondaries 24, 25, 26 through a rectifier A so as to be energized with a substantially continuous current when earth faults are present, the polarising winding being energized by a local D.C. source 23. The relays 18 are only operated however, if the restraining windings 22 remain unoperated, the energization of these windings being under the control of the carrier-wave currents. These currents are transmitted from both ends of the protected section by transmitters each fed through converting apparatus B of the type already specified which derives current from the local D.C. supply 23. The transmitters and receivers C are coupled to the protected section by well-known forms of coupling members, the carrier currents being confined within the section by trap circuits 27. The transmitters and receivers come into operation only on alternate half-cycles of the fault current and in the case of an earth fault outside the controlled section in which the instantaneous values of the fault current are substantially the same at both ends of the section ; the transmitters at the ends transmit alternately. Consequently both receivers receive and the restraining winding 22 of each associated relay is energized from the source 23 and neither circuit-breaker 15, 16 is tripped. With an earth fault within the protected section, however, the transmitters transmit simultaneously since the earth fault current adjacent one transmitter differs in phase by 180‹ from that adjacent the other transmitter. Consequently neither receiver receives and the restraining winding 22 of each tripping relay remains de-energized, so permitting the operating windings of these relays to produce closure of their contacts and complete the tripping circuits of circuit-breakers 15, 16. To limit the voltage supplied to the rectifier supplying the operating windings 21 the parallel-connected series transformers 24, 25, 26 may be shunted by the primary windings of transformers 29, the secondary windings of which are connected to voltage-limiters such as resistances having negative volt-ampere characteristics of the type described in Specification 292,110, [Class 37, Electricity, Measuring &c.], Filter circuits 30, 31 prevent H.F. oscillations from passing into the series transformer windings.